1. Field of the Invention
Embodiments of the present invention generally relate to light-emitting semiconductor devices and, more particularly, to packaging such devices with multiple encapsulation layers in an effort to produce uniform white light.
2. Description of the Related Art
Many techniques exist to emit white light from semiconductor devices, such as light-emitting diodes (LEDs). Some of these include combining the outputs of individual red, green, and blue LEDs; combining a blue LED with yellow phosphor or green and red phosphor; and combining an ultraviolet LED with red, green, and blue phosphor. The simplest technique, or at least the one with the least elements, involves employing a blue LED combined with a layer or coating of yellow phosphor, as disclosed in U.S. Pat. No. 5,998,925, entitled “Light Emitting Device Having a Nitride Compound Semiconductor and a Phosphor Containing a Garnet Fluorescent Material,” and illustrated in FIG. 1.
FIG. 1 depicts a lead-type light-emitting diode (LED) 100 where the light-emitting component 102 is installed on a mount lead 104. An n-electrode and a p-electrode of the light-emitting component 102 are connected to the mount lead 104 and a second lead 106, respectively, via wires 108. A cup of the mount lead 104 is filled with a coating resin 110 that contains a specified phosphor to cover the light-emitting component 102. The leads 104,106, light-emitting component 102, and the coating resin 110 are encased in a molding material 112, which protects the light-emitting component 102 and may function as a lens to focus or diffuse the light emitted by the LED 100. When the LED is forward biased, light emitted by the light-emitting component 102 excites the phosphor contained in the coating resin 110 to generate fluorescent light having a wavelength different from that of the light-emitting component's light, so that the fluorescent light emitted by the phosphor and the light-emitting component's light that is output without contributing to the excitation of the phosphor are mixed and output. Thus, when the light-emitting component 102 employs a gallium nitride (GaN) compound semiconductor and the coating resin 110 includes a garnet phosphor activated with cerium, blue light is emitted from the light-emitting component 102, and some of the light excites the phosphor to produce yellow light. The blended combination of blue and yellow light essentially produces white light.
However, the white light produced by conventional light-emitting semiconductor devices employing a blue LED and a yellow phosphor exhibits a color ring phenomenon, where the periphery of the emitted light appears more yellow and the middle appears bluer. Referring now to FIG. 2, the optical spectrum 200 is not uniform and does not have the broadband characteristic of true white light. One can easily discern the sharp peak 202 at a wavelength of approximately 450 nm arising from the blue light-emitting component 102 and the less intense, broader bandwidth yellow component 204 having a center wavelength of approximately 565 nm from the excited phosphor in the coating resin 110. When the blue light excites the phosphor at different angles, the white light spectrum changes due to the blue light intensity (which is proportional to the cosine of the emission angle), the phosphor concentration, and the phosphor thickness, thus yielding the color ring.
To measure the uniformity of the emitted white light, the variation in the correlated color temperature (CCT) may be used. The color temperature of a light source is determined by comparing its hue with a theoretical, heated blackbody radiator. The Kelvin temperature at which the heated blackbody radiator matches the hue of the light source is that source's color temperature. An incandescent light is very close to being a blackbody radiator, but many other light sources, such as fluorescent lamps, do not emit radiation in the form of a blackbody curve and are therefore assigned what is known as a correlated color temperature (CCT). The CCT is the color temperature of a blackbody which most closely matches the light source's perceived color. The higher the Kelvin rating, the “cooler” or more blue the light. The lower the rating, the “warmer” or more yellow the light.
By measuring the CCT at different light emission angles and comparing this variation among different white-light-emitting devices, the uniformity of the white light produced can be quantified. A blue LED with a coating resin of yellow phosphor, such as the LED 100 of FIG. 1, may have a typical CCT graph 300 as shown in FIG. 3 where the CCT curve 302 varies from approximately 5800 K to 7200 K across a 1400 (±70° from the center light-emitting axis of the LED) range of light emission angles. Because of the color ring, the CCT is higher in the center than in the periphery, where the light tends to be more yellow.
To reduce the color variation and improve the uniformity of the emitted white light, manufacturers have tried packaging the light-emitting diode die (LED chip) in various combinations of encapsulation materials, such as those disclosed in U.S. Published Patent Application No. 2005/0221519, entitled “Semiconductor Light Emitting Devices Including a Luminescent Conversion Element and Methods for Packaging the Same,” filed Feb. 10, 2005. FIG. 4 illustrates one such packaged light-emitting diode (LED) device 400, where a light-emitting device 402 is coupled to a substrate 404 and disposed in the cavity of a reflector cup 406. A first encapsulant material 408 is dispensed above the light-emitting device 402 and cured, for example, by heating. A second encapsulant material 410 including a luminescent wavelength conversion material, such as a phosphor, is dispensed into the cavity above the first encapsulant material 408. Having a biconvex, plano-convex, or concavo-convex shape, the second encapsulant material 410 is shown completely covering the first encapsulant material 408. A third encapsulant material 412 having no or a low concentration of luminescent material is dispensed above the second encapsulant material 410, and a lens 414 is disposed in the third encapsulant material 412 before curing.
The resulting CCT polar plot 500 for a semiconductor light-emitting device with a luminescence conversion element, such as the packaged LED device 400 of FIG. 4, is illustrated in FIG. 5. The CCT curve 502 illustrates that near the central light-emitting axis (corresponding to a light emission angle of 0°), the CCT is over 7000 K. Around 45° and 67.5°, the CCT is close to (and in some cases, less than) 6000 K leading to a variation of around 1000 K for a 140° (±70° from the center light-emitting axis) range of light emission angles. A CCT variation of 1000 K may not be considered as very uniform white light by those skilled in the art.
Accordingly, what are needed are methods and apparatus to reduce the color variation (i.e., increase the uniformity) in white light emitted from semiconductor devices.